We are continuing the study of (1) the mechanism of transcriptional control and (2) the detailed post-transcriptional events of primary RNA processing and decay in the regulation of gene expression of the major leftward operon of bacteriophage lambda. In the absence of N product, the RNA derived from the gene N segment (pL-N-t-L1) of the leftward operon has been identified in vivo as a 12-14 S (12) RNA transcript containing all RNA sequences from the promoter (pL) to the first transcriptional termination signal (tL1). Immediately after completion of synthesis, the 12 RNA transcript undergoes RNase III-specific processing at the 5'-proximal end thus releasing the 5'-terminal 1 degree 1 and the next adjacent 11 RNA species. The pL-distal RNA downstream from 11--to-tL1 is differentially very labile, possibly due to rapid decay via a 3'-exonuclease pathway. In the presence of N product, the 12 RNA transcript is elongated (antitermination) to the att region of the operon yielding the N-dependent 1'3 RNA transcript (pL-to-att). Before 1'3 RNA synthesis is completed, RNase III releases the 5'-proximal 1 degree 1 and 11 RNA species and, in addition, appears to cleave the 1'3 RNA at or near the tL1 signal. This latter event leads to the rapid release and subsequent decay of the remaining RNA of the 12 segment. The remaining RNA segment of the 13 RNA transcript from tL1-to-att (13 RNA) appears to undergo progressive secondary endonucleolytic cleavage events followed by 3'-exonuclease decay. In addition to the tL1 signal, we have identified a second leftward termination signal (tL2) downstream in the viscinity of the exo region. The identity of tL2 as an N-dependent transcription termination signal remains to be established.